The invention relates to multilayered plastic containers having improved resistance to oxygen permeation and to compositions and processes for production of multilayered plastic bottles.
In order to be technically acceptable, beer containers (glass, metal, or plastic) must maintain the beer contained therein in a near oxygen free environment. A generally accepted industry standard is considered to be a maximum of 1 ppm oxygen ingress into the bottle over the planned shelf life of the bottled beer. Further, not only must oxygen be excluded from the bottled beer, but the egress of carbon dioxide from the beer out through the bottle walls must also be eliminated or at least contained to defined standards.
Oxygen may be present in bottled beer from at least three separate sources. In some instances, unwanted oxygen (from air) is not completely eliminated from the space above the liquid in the beer bottle during the bottle filling process. Oxygen arising from this source is known as head space oxygen. Even beer packaged in cans is susceptible to the presence of head space oxygen. In conventionally capped glass beer bottles, oxygen may enter the bottle during storage by permeation through the medium used as the gasket in the crimped bottle crown. A third source of oxygen in bottled beer is specific to the use of plastic bottles. Oxygen, from air, has the ability to permeate many conventional bottling polyesters and end up inside the bottle cavity. Also, for plastic bottles, oxygen may be dissolved or adsorbed in the plastic. Oxygen dissolved in or adsorbed on the plastic bottle walls may be desorbed and end up in the bottle cavity. Such desorbed oxygen is indistinguishable from head space oxygen once inside the bottle cavity, except that it should be viewed as a possible continuing source of oxygen which must be consumed or depleted. For the purposes hereof, desorbed oxygen will be considered to be a factor which contributes to head space oxygen. Oxygen dissolved in the plastic wall is indistinguishable from oxygen attempting to permeate through the plastic bottle walls. For the purposes hereof oxygen dissolved in the plastic bottle walls will be considered the same as oxygen attempting to permeate the bottle walls. In summary, then, beer packaged in metal cans is generally at risk only from head space oxygen. Beer in glass bottles is generally at risk from head space oxygen and also from oxygen permeation through the bottle closure means, especially crimped crown gaskets. Beer in plastic bottles is at oxygen risk from the two sources noted above and also from permeation of oxygen through the bottle wall into the bottle cavity. These considerations also apply to other products packaged in cans and bottles though the effects of oxygen can vary considerably depending on oxygen sensitivity of the product.
While the bottling of beer in plastic bottles is still in its infancy, the above recitation as methods for unwanted oxygen to be present in a plastic bottle cavity are well documented in the art, not only for bottling applications having oxygen requirements as rigorous as those for beer but also for applications less stringent than those for bottling beer. Attempts to overcome these problems for plastic bottles have often involved the use of multilayered bottles where at least one of the layers comprises a polymer (such as ethylene vinyl alcohol copolymer, EVOH) having superior passive resistance to oxygen permeation as compared to the bottling polyester which is usually polyethylene terephthalate (PET). There are disadvantages to such approaches including the following: (1) the bottles are no longer suitable for recycle with other polyester (PET) bottles because of the presence of a second and incompatible polymer (EVOH), (2) the bottles tend to delaminate at the PET/EVOH interfaces, although such delamination may be somewhat diminished (at additional expense) by the use of adhesive tie layers, (3) the differences in melting points and other physical properties between PET and EVOH cause numerous problems in the bottle fabrication process, and (4) use of a passive oxygen barrier, such as an EVOH layer, tends to keep head space oxygen trapped within the bottle cavity instead of eliminating it.
This invention addresses these and other problems related to prior art efforts to manufacture zero and near zero oxygen permeation plastic bottles.
In a broad sense, therefore, this invention relates to novel bottles and a process for the production of multilayered substantially zero oxygen permeation plastic bottles. Substantially zero oxygen permeation means that the oxygen which finds its way to the bottled product is an amount which is only barely measurable with instruments which measure such permeation. In the absence of a specific amount of oxygen, substantially zero oxygen permeation will be considered to be 1 ppm of oxygen, in terms of the weight of the bottled product, for the target shelf life of the bottled product. The multilayered plastic bottles of this invention are suitable for recycle with other polyester bottles, have excellent rigidity, have good clarity when such clarity is desired, resist delamination, do not need tie layers, and also have the ability not only to keep oxygen (from air) from entering the bottle cavity but also have the ability to consume or deplete the presence of unwanted oxygen in the bottle cavity. The novel bottles of this invention involve the use of modern multilayer bottle making processes and equipment in conjunction with deployment of at least one layer (of the multilayered plastic bottle) which comprises a copolyester oxygen scavenging formulation which is an active oxygen scavenger. Active oxygen scavengers consume (or otherwise deplete) oxygen from a given environment. As noted in the co-pending application, a zero oxygen permeation multilayer bottle will have enough oxygen scavenging capacity to consume any unwanted (head space) oxygen in a bottle cavity and still have enough capacity remaining to consume oxygen at the rate at which it reaches the scavenger layer from air external to the container for the necessary shelf life of the filled bottle.
Applicants"" oxygen scavenger systems are block copolycondensates comprising predominately poycondensate segments and an oxygen scavenging amount of polyolefin oligomer segments. Predominately means that at least 50 weight % of the copolycondensate may be attributed to polycondensate segments. The preferred polycondensate segments, especially for bottling use, are polyester segments. For layers in multilayered bottles in which some of the layers are PET, or modified polyesters such as PETI, PETN, APET, PETB and/or PEN, segments of the block copolyester comprising these same polyesters are especially preferred. A primary reason is that the copolyesters most closely emulate the polyester from which its polyester segments are derived. The polyesters recited above and the various modified bottling polyesters considered safe for use with food as listed in 21 CFR xc2xa7 177.1630 are the polyesters of choice for bottles because of their clarity, rigidity, and long history of usage for food and beverage storage. It will be understood that the many references to PET made in this specification shall (unless otherwise indicated) encompass not only PET, but shall also encompass PET as is commonly used in various modified forms for bottling including, but not limited, to the list of modified polyesters recited above and subsequently defined in greater detail in this specification.
The polyolefin oligomer segments are prepared for copolycondensation by first functionalizing the polyolefin oligomer segments with end groups capable of entering into polycondensation reactions. This is an important feature because the polyolefin oligomers are, in effect, addition polymers. Functionalization of the polyolefin oligomers with end groups affords a convenient method for incorporation of addition polymer segments into a copolycondensate. A preferred polyolefin oligomer is polybutadiene (PBD) because it has good oxygen scavenging capacity and reacts quickly with oxygen especially in the presence of a transition metal catalyst, such as cobalt, and in the presence of benzophenone, or both cobalt and benzophenone.
One of the salient features of the oxygen scavenging copolyesters of this invention is their ability to scavenge oxygen in the presence or absence of water or even moisture. While much of the discussion of this disclosure is focused on zero oxygen permeation beer bottles, many other materials are suitable for being bottled and/or packaged in the zero and near zero oxygen permeation packaging environments envisioned and encompassed by this invention. Examples, other than beer, of perishable food and beverages for which a zero oxygen permeation bottle, jar, or specialized container would be desirable are well known and include wines, fruit juices, beverage concentrates, isotonics, flavored teas, tomato based products such as catsup, salsas, and barbecue sauces, vinegar, mayonnaise, baby food, nuts, and dry foodstuffs of all varieties. Non-food items requiring zero oxygen permeation packaging would include oxygen sensitive electronic parts.
One reason that the timing of this invention is so appropriate has to do with the recent trend in the food and beverage industries of providing consumer information regarding product freshness. Whether legislated or voluntary, it has become rather standard practice in the food and beverage industries to provide an un-coded, easily understood xe2x80x9csell byxe2x80x9d, xe2x80x9cuse byxe2x80x9d, or xe2x80x9cbottled onxe2x80x9d date clearly printed on the bottle or package. This long felt need to satisfy consumer awareness of product freshness has recently been very well exemplified by a major USA brewery advertising campaign featuring their so-called xe2x80x9cborn onxe2x80x9d date for bottled beer. These consumer information data on packages and bottles assist consumers in their determination of product suitability and freshness. These data are also of value in the application of this invention since knowledge of the target shelf life for a given product permits easy calculation of oxygen scavenging capacity required to sustain zero (or near zero) oxygen permeation for the maximum planned shelf life.
The adjustment of the oxygen scavenging capacity of the bottles of this invention to ensure zero oxygen permeation vary not only by product but also within a given product line. In a paper entitled REQUIREMENTS FOR PLASTIC BEER PACKAGES presented at the xe2x80x9cFuture-Pak ""96xe2x80x9d conference by Dr. Nick J. Huige of the Miller Brewing Company, it was disclosed that for US domestic beers, a maximum ingress of 1000 ppb (1 ppm) over 120 day shelf life when stored at 75xc2x0 F. (24xc2x0 C.) was generally recognized as the industry standard. It is a common practice to pull any beer over 120 days old (i.e., 120 days since bottling) from the retailers"" shelf and destroy it. This is done for many US beers not only because of the possible presence of oxygen, but also because of other changes which occur once beer is bottled, especially the appearance of a musty or skunky character. Huige also estimates that about 95% of the beer from major US breweries reaches consumers within 60 days of bottling. But in keeping with the industry standard, a planned shelf life of zero oxygen permeation for 120 days at 75xc2x0 F. (24xc2x0 C.) is a realistic target for bottling beer from major US breweries.
For US micro-breweries and European beer makers, the requirements may be totally different. For US micro-breweries, it is unlikely that 95% of the product reaches consumers within 60 days of bottling. Also, European beer makers (and to a lesser extent, US micro-breweries) consider it desirable for bottled beer to take on what is characterized by beer tasters as a xe2x80x9cpapery/cardboardxe2x80x9d flavor, a characteristic associated with at least partial oxidation of the beer in the bottle. This is strictly an undesirable attribute for the lighter bodied, more delicately balanced American beers. From these few considerations, it becomes obvious that setting the acceptable oxygen permeation rate, including zero oxygen permeation shelf life requirements, is not always a simple matter. But it can be predicted and calculated in most cases and empirically derived in other cases. Once known, the methods of adjusting the oxygen scavenging capacity and/or zero oxygen permeation shelf life required of the bottle may be achieved by one or a combination of several of the methods of this invention as disclosed later in detail.
Published PCT Application (WO 96/18686 published on Jun. 20, 1996) discloses the use of aliphatic polyketone materials as oxygen scavengers. This reference has no examples of fabricated zero oxygen permeation bottles. There is no experimental data in the reference other than primary aliphatic polyketone permeability coefficients, and it is unclear whether these data were experimental or supplied by the resin manufacturer. The oxygen scavenger performance described in the reference is insufficient by several orders of magnitude to maintain zero oxygen permeation, i.e., the scavenger capacity is insufficient to consume the oxygen at the rate at which it reaches the scavenger layer by permeation through the outer PET layer.
Japanese patent document 3-275327 laid open on Dec. 12, 1991 describes a blown bottle having walls which include an xe2x80x9can oxygen impermeablexe2x80x9d layer of a xe2x80x9cmethoxyarylenediaminexe2x80x9d. The data in this reference show a reduction in oxygen permeation down to 28% of the amount passed using PET only bottle walls. This amount is inconsistent with the goal of this invention which is zero oxygen permeation.
A single layer (homogeneous and monolithic) oxygen scavenging bottle wall is disclosed in European Patent Application EP 380,830 published on Aug. 8, 1990. This reference discloses OXBAR bottle walls (suitable for making beer bottles according to the teaching). OXBAR is a blend of about 96 wt % true PET, about 4 wt % MXD6, and a solution of C8-C10 cobalt carboxylates having about 10 wt % cobalt as metal deployed so as to provide about 50 ppm cobalt in terms of the weight of the blend. MXD6 is a polyamide prepared from equi-molar amounts of adipic acid and metaxylene diamine. According to the reference, the presence of the MXD6 not only serves as an oxygen scavenger but also enhances the ability of the PET to retard egress of CO2 from the bottle cavity out through the bottle walls. Any bottles made according to this reference would have some serious deficiencies including, among others, (1) loss of recycle possibilities, (2) higher cost since the entire bottle consists of oxygen scavenger material, (3) no opportunity for use of recycled PET since the homogeneous walls are in contact with the bottled product, (4) potential excess leaching of cobalt into the bottled product, (5) no means to efficiently and cost effectively adapt bottle oxygen scavenging capacity to required shelf life, and (6) rapid loss of oxygen scavenging capacity (even in the preform stage) because of blatant attack of oxygen from air directly on the oxygen scavenger moiety. While not disclosed in the reference, applicants have speculated on the effectiveness of a bottle comprising an outer layer of PET, a middle layer of OXBAR, and an inner layer of PET. The cost (very thick layer of OXBAR needed to supply necessary oxygen scavenging capacity) and recycle issues would still be present in such an embodiment.
The only significant disadvantage of using multilayered bottle walls is that more complex bottle making machinery is required to form the multiple layers. The advantages which accrue from the use of multilayer bottle walls far out weigh the single advantage of simpler processing associated with a homogeneous single layer bottle wall. Typically, bottle wall embodiments of this invention are three layer constructions of Layers A-B-C. Layer A is the outer layer forming the exterior of the bottle and is in contact with the outside air. Layer B is the oxygen scavenger layer. Layer C is the inner layer and defines the bottle cavity. Among the advantages of such a multilayered construction are (1) ability to use recycled PET in Layer A, (2) ability to dilute (within limits) the scavenger layer, Layer B, with recycle or virgin PET so as to easily and cost effectively adjust zero oxygen permeation capacity to the planned shelf life of the product, (3) isolation of the packaged (bottled) product from the oxygen scavenging layer via Layer C (Layer C is normally virgin PET), (4) isolation of the oxygen scavenging layer from oxygen in air because of the presence of outer Layer A, and (5) retention of recycle ability as the multilayer bottles of this embodiment of the invention are typically over 99.6% PET and PET segments. Also envisioned is the use of a 5 layer bottle wall of the type A/B/Axe2x80x2/B/A where A is PET, B is the scavenger layer(s) either neat or diluted and Axe2x80x2 is also PET, especially recycled PET.